A general process for producing a turbine or compressor blade having a plurality of passages therein relatively close to the surface for the passage of fluid therethrough for cooling the blade is set forth in U.S. Pat. No. 2,641,439 - Williams. An improvement on the Williams process is described in U.S. patent application Ser. No. 296,756--Grondahl (now abandoned) filed Oct. 12, l972. The Grondahl application is assigned to the assignee of the instant invention and both the Williams' patent and the Grondahl application are incorporated by reference.
The Williams patent provides that in forming a turbine blade the surface of the core has formed therein a plurality of grooves separated by ridges. Cover for these grooves to provide closed channels for the conduct of cooling fluid during use is provided by plating a layer of metal over the bucket core, the grooves first being filled with a material such as aluminum, cadmium or a graphite-containing wax. Thereafter, one or more layers of metal are deposited thereover by electrodeposition forming the skin.
Next, the skin having been deposited and bonded in place, the patent describes the removal of the filler material to leave open passages defined by the grooves and the metal skin deposited thereover. According to the directions, when the filler material is aluminum, it would be removed by reaction with caustic soda. When the filler material is cadmium, it would be removed by heating to vaporization. When the filler material is a graphite-containing wax, it would be removed by melting.
The Williams' process can be successfully employed when the cooling channels are substantially larger in cross section than the capillary-size passageways employed in the liquid cooled turbine buckets described in U.S. Pat. No. 3,446,481--Kydd (incorporated by reference). It is particularly important that no plugging of the capillary size coolant channels occur to retard the flow of liquid coolant therethrough, because such turbines are intended to operate at very high temperatures and failure to cool any portion or portions of the blade surface can have very serious consequences.
In both the Williams patent and the Grondahl application the airfoil skin is produced in situ, as by electroplating. However, when the normal hydrogen or vacuum brazing process is employed to braze preformed skins to airfoil cores, the process renders all of the surfaces present clean (free of oxides or other contaminants). The braze material is free to and does flow over these surfaces. Blockage of cooling channels can occur by entry of the molten brazed material into the coolant channels. It is to the latter problem that the instant invention is directed.